Introduction to the Petromyzontiformes

Lampreys

The long, eel-like fish drinking the blood of this lake trout is one of
nearly fifty species of lampreys, a group of jawless fishes
found in temperate rivers and coastal seas.
Some species live in fresh water for their entire lives. Others, like the
sea lamprey (Petromyzon marinus) shown here, are anadromous.
That is, they are hatched and grow in fresh water, migrate to the ocean to
mature, and then return to fresh water to spawn.

Lamprey eggs hatch into small larvae, known as ammocoetes, which
are not predators at all; they lack the sucker mouths of the adults,
and feed by producing strands of mucus and trapping
food particles. The ammocoetes stage may last up to seven years before
its metamorphosis into an adult. Adult lampreys live for a year or two
before spawning, and then die soon afterwards. The adults bear seven
separate gill openings and have an internal skeleton of cartilage.
Unlike hagfishes, with which they were once
classified, lampreys have a complete braincase and rudimentary true vertebrae.
Uniquely among living vertebrates, lampreys also have
a single "nostril" on the dorsal side of the head -- a feature they share with
various fossil jawless fish, which had a similar opening.

Lampreys have a round, suckerlike jawless mouth filled with rows of
horny teeth, and a rasplike "tongue." (See picture at right)
An internal ring of cartilage supports the rim of the mouth.
Although lampreys sometimes prey on small invertebrates, they are better known
as predators on fish. Fastening on to living fish, lampreys rasp into the flesh
and feed on the body fluids. A fish attacked by lampreys may be severely
weakened or even killed.

There have always been native freshwater lampreys in North American waters.
But since 1835, the sea lamprey has been spreading through manmade canals
into the Great Lakes of northeastern North America; it is now found in
all five lakes and in several of the rivers that flow into them. In the
1940s and 1950s, the sea lamprey caused serious damage to the
fisheries of the Great Lakes. It is now the subject of control efforts
by U.S. and Canadian governmental agencies, and the Great Lakes fisheries
have recovered.

Because they lack mineralized tissues such as bone, lampreys are rare
as fossils, and their early evolutionary history is still poorly known.
Only three definite lamprey species are known from the fossil record, all
from the Pennsylvanian (Late Carboniferous) of northeastern Illinois. These
are very similar to living lampreys, and suggest that the group has not
changed very much in the past 300 million years. A few other unarmored,
jawless fossil fishes (traditionally classified in the Anaspida)
from elsewhere in the world are now thought by some to be members of
the Petromyzontiformes, or at least close relatives.
The earliest likely candidate is the early Silurian
fossil Jamoytius, which may have had a cartilage ring supporting a
round mouth, like living lampreys.